The use of high-solids coating compositions to reduce the emission of organic compounds when a coating material is applied is to be welcomed from an environmental view-point.
It is known that binders for such coatings can be prepared by polymerization in bulk, in which one component, for example, a glycidyl ester or a maleate is introduced as initial charge at the beginning of the polymerization and this ester or this monomer is incorporated fully into the copolymer in the subsequent course of the polymerization, with the addition of (in the case of the maleate, further) monomers. See EP-A 0 027 931, EP-A 0 056 971 and DE-P-43 24 801.2.
In the case of bulk polymerization with a glycidyl ester, the ester is introduced as initial charge and then reacted completely with monomers of which at least one contains at least equimolar quantities of acid and/or anhydride groups. The monomers with anhydride groups are at least partially hydrolyzed. Thus, in addition to the polymerization, a reaction of the epoxide groups with the carboxyl groups takes place in which in each case one ester group and one secondary hydroxyl group are formed.
The advantage of bulk polymerization, in comparison with polymerization in a solvent, lies in the freedom of choice of the solvents used for dilution after the end of the reaction. By this means, it is possible to formulate even for very high concentrations of binder in the solvent. A further advantage is that low-boiling solvents, for example, n-butyl acetate, can be used to dilute the polymer, and that high-boiling solvents which in some cases are toxicologically objectionable, for example, alkylated aromatic compounds, can be substantially avoided.
The advantage of this bulk polymerization, in comparison to a pure mass polymerization, is that it enables good dissipation of the heat of reaction and therefore a safe reaction procedure. The correct choice of the reaction parameters, in particular of the reaction temperature of the initiator, then leads to products with low viscosities and thus to high-solids contents in a finished coating formulation, as described in EP-A 0 027 931 and in DE-P-43 24 801.2.
From a point of view of applications, high-solids products with a high proportion of acid and/or anhydride groups are desirable because, for example, they result in a sharp acceleration of curing in automotive refinishing. Even quicker dust-drying and freedom from tack with the same pot life are requirements which are frequently made by the vehicle finishing workshops.
For the basecoat/clearcoat process in the production-line finishing of automobiles it is equally important to use clearcoats having as low as possible a content of organic solvents. Moreover, a further aim is to replace high-solvent by low-solvent solid-color topcoats.
Owing to their application, these production-line or OEM coating materials are baked at an elevated temperature. They are intended after baking to give paint films which are free from boil marks and are of good appearance with good resistance properties. Frequent complaints from the automobile manufacturers concern, in particular, inadequate resistance to sulfuric acid and to xylene and in the cation-anion test.
However, attempts to raise the reactivity of the binder by increasing the ratio of the number of acid and/or anhydride groups to the number of epoxy groups in the glycidyl esters, in a procedure according to the above-mentioned patent applications, lead to products of increased solution viscosity, which are therefore no longer high-solids products. This also occurs in the case of corresponding products which have tert-butyl groups in the polymer. As is known, the tert-butyl group may be oxidized during or after the polymerization to give an acid group.
These higher solution viscosities can apparently be attributed to the increased possibility for the formation of both inter- and intramolecular hydrogen bonds (secondary bonding forces), owing to the simultaneous presence of high amounts of hydroxyl and acid groups in the copolymer. In addition, esterification and/or transesterification reactions under these polymerization conditions may also contribute to the high solution viscosities of the hydroxyl- and carboxyl-containing copolymers.
The content of excess free acid and/or anhydride groups in the copolymer is therefore limited in the preparation of high-solids binders in accordance with the above-mentioned patent applications.